Modulation is the process of encoding information onto a bandpass signal for purposes of transmission via a selected medium. Many modulation techniques are known for encoding data, including single-carrier and multi-carrier methods, where the particular method chosen depends upon the application and, the selected medium of transmission. One such multi-carrier method used for wireless communications is orthogonal frequency division multiplexing (OFDM). For example, the Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11a standard defines multiple data rates in the 5 Gigahertz (GHz) frequency band employing orthogonal frequency division multiplexing (OFDM) for use with Wireless Local Area Networks (WLANs). OFDM is an encoding process performed in a transmitter in which data is mapped onto multiple subcarriers, which are then combined into one baseband signal using Inverse Fast-Fourier Transform (IFFT). The encoded baseband signal is ultimately up-converted to the appropriate transmission radio frequency (RF), such as, for example, 5 GHz for 802.11a. The reverse process is performed at the receiver, which down-converts a received RF signal to a baseband signal and then demodulates the baseband signal to recover the original data.
The up-conversion and down-conversion process may be performed in multiple steps involving at least one intermediate frequency (IF). In one exemplary transmitter configuration, an OFDM encoded signal modulates a first carrier IF of approximately 20 megahertz (MHz), and then the modulated signal is up-converted to a second, higher IF of approximately 800 MHz, and then up-converted to the final RF of approximately 5 GHz for transmission through a wireless medium via an antenna. The initial up-conversion to the lower IF may be a modulation process in which a carrier signal is modulated by the baseband data. One such modulation process is known as Weaver modulation in which a mixer is used to combine the baseband signal with a sinusoidal signal at the desired IF resulting in a sideband signal at IF. To avoid various radio impairments, such as DC offset and flicker noise, the initial encoding and modulation process to the lower IF is typically performed digitally using a baseband processor or the like. The raw data is encoded using OFDM or the like, interpolated to increase bandwidth and sample rate and to maintain signal fidelity, and then modulated to the first IF using an appropriate scheme such as Weaver modulation. A digital to analog converter (DAC) converts the signal to an analog signal, which is then processed using additional filters and mixers to produce the appropriate RF signal for transmission. A corresponding receiver employs mixers and filters to down-convert a received RF signal to the lower IF, and an analog to digital converter (ADC) to convert the analog IF signal to digital format. The IF signal is then digitally demodulated to a baseband signal, and one or more decimators reduce the sample rate to the appropriate level for further baseband processing to retrieve the original data.
Encoding schemes such as OFDM result in complex digital data including real and imaginary digital samples in the transmitter. The interpolation process is performed on the complex information, which had typically required both real and imaginary circuit portions. The Weaver modulation has required a complex multiplier or the like to combine complex interpolated samples with complex phase-shift values resulting in a complex IF signal. Only after the complex IF signal is generated can the imaginary values be stripped out resulting in real-only data values. A negative-frequency mirror image is created by saving only the real part of the signal, but this undesired signal energy can be removed by analog filtering in the radio. In the receiver, real-only samples are sampled by the ADC, but were typically converted to complex samples by the Weaver demodulation process. The digital decimators, therefore, had to be implemented to process both the real and imaginary values to provide accurate data for decoding within the baseband processor. In this manner, the encoding, interpolation and modulation process as well as the demodulation and decimation process has required a significant number of components to perform the desired functions, including shift registers, complex adders, complex multipliers, etc. This has resulting in a relatively large number of gates, nets, cells and total silicon area to implement a baseband processor in silicon. The additional number of components and resulting large size of the circuitry had further resulted in significant power consumption.
It is desired to provide more efficient modulation and demodulation solutions, such as by simplifying or otherwise reducing circuitry and resulting power consumption.